1. Field of the Invention
The present invention relates to an optical transmission system utilizing wavelength multiplexing technology and in particular relates to an optical transmission system having stable transmission characteristics in the overhead portion of bit patterns comprised of a plurality of bits.
2. Description of Related Art
Along with the expanded demand for communications, greater demands are also being made for increased transmission capacity along each optical fiber. Wavelength division multiplexing is being developed to a practical level to provide greater transmission capacity. In order to provide greater transmission capacity by means of wavelength division multiplexing (WDM), problems such as attaining high speed signals, a high density placement of optical signals on a wavelength, an expanded bandwidth for the wavelength region being used, a high power optical signal and suppression of the non-linear optical effect must be dealt with.
The non-linear optical effect is an optical phenomenon caused by the non-linear response of matter and is found only in light that is nearly monochrome and has directivity such as laser light. The following are conditions known up until now under which the non-linear optical effect is prone to occur: (1) Relatively large optical power (2) Transmission in low-dispersion range of a fiber transmission path, (3) Narrow wavelength interval (4) Bit pattern matches with other channel intervals.
When using wavelength multiplexing such as WDM technology, the non-linear optical effect is easily prone to occur when the bit or bit pattern of an optical signal is phase-matched.
FIG. 29 is a block diagram of an experimental optical transmission system showing the non-linear optical effect due to phase-matching of the bit. This optical transmission system is comprised of a standard semiconductor laser 12 to output a standard laser beam 11 of a standard wavelength xcexs, and a reference semiconductor lasers 141 . . . 14N to output a reference laser beam 131 . . . 13N on a wavelengths xcex1 . . . xcexN.
The standard laser beam 11 output from the semiconductor laser 12 is input to an optical modulator 16 and modulated by a specified pulse pattern output from a pulse pattern (PPG) generator 18. After modulation, the laser light 19 is input to one input of an optical coupler 21. The reference laser beam 131 . . . 13N output from the reference semiconductor lasers 141 . . . 14N, is input to a wavelength multiplexer 24 and subjected to wavelength multiplexing. The laser light 23 is input to an optical modulator 24 after multiplexing, and modulated by a specified pulse pattern 26 output from a pulse pattern generator 25.
After combining in the optical coupler 22, the laser light 28 is input to a bit correlation eliminator (DCL) 29, then input to an optical amplifier 31, and amplified and sent to a first transmission fiber 31,. This laser light passes through the transmission path optical fibers 321, . . . 32k, 32(k+1) as well as the optical amplifiers 312, . . . 31k, 31(k+1) and is transmitted to the optical band path filter of the other (remote) party. The wavelength of the standard laser 11 and the reference laser beam 131 . . . 13N is the wavelength within the gain-bandwidth of the optical amplifiers 312, . . . 31k, 31(k+1). After only passing the wavelength xcex on the receive side, the laser light 35 is input to a receive circuit 36. This receive circuit 36 is connected to an error detector 37.
In this kind of experimental system, modulation is applied simultaneously on and overhead comprised of identical bit patterns for all wavelengths xcexs, xcex1 . . . xcexN, phase-matching conditions then provided, and on/off settings made on the bit correlation eliminator (DCL) 29 in order to eliminate bit correlation in this state, and the propagation characteristics measured for respective states. The bit correlation eliminator (DCL) is comprised of dispersion compensating fiber and has a dispersion characteristic of approximately xe2x88x92400 ps (picoseconds) per nanometer on the wavelength bandwidth being used.
In this experiment, the type N for the reference laser beam 131 . . . 13N wavelengths is set as xe2x80x9c11xe2x80x9d and the figure K for the optical amplifiers 312, . . . 31k, 31(k+1) is set as xe2x80x9c3xe2x80x9d. Also, DSF (dispersion shifted fiber) 8.0 kilometers each were used in the respective transmission path optical fibers 321, . . . -324. The output levels of the 312, . . . 314 were all +5 dBm per 1 channel.
Measurement results for the experimental optical transmission systems are shown in FIG. 30 through FIG. 32. Of these figures, FIG. 30 shows the code error rate during receive. The test points with X marks 41 in this figure, indicate the bit correlation eliminator 29 is off and bit correlation is not being canceled. In this state, bit phase matches are present. The test point with the xe2x80x9c∘xe2x80x9d marks 42 in contrast, indicate that the bit correlation eliminator 29 is on and bit correlation is being canceled. The bit phase matches are greatly reduced at this time.
FIG. 31 shows the receive optical waveform when the bit correlation eliminator 29 is off and bit matching of each wavelength xcexs, xcex1 . . . xcexN, is being performed. In FIG. 32 the receive optical waveform is shown, with the bit correlation eliminator 29 on and bit matching of each wavelength xcexs, xcex1 . . . xcexN, is not being performed. In both FIG. 31 and FIG. 32, a waveform of 2.5 Gb/s (gigabytes per second) is shown.
The horizontal axis in FIG. 30 shows the average power of the light level being received. Usually, the bit error rate on the vertical axis decreases when the optical signal receive level is raised. However, with the bit correlation eliminator 29 off, the bit error rate will not fall below 10xe2x88x925, even if the receive level (power) is increased and a higher SIN (signal-to-noise) ratio for the optical signal as shown in FIG. 30. This state is due to a non-linear optical effect in the receive optical signal itself. If the bit correlation eliminator 29 is on however, the extent of bit phase matching decreases greatly. Therefore, the error rate will be reduced to a sufficiently small value if the receive level (power) is increased and the optical signal given a higher S/N (signal-to-noise) ratio, as clearly shown by the one-bit interval change in the waveform in FIG. 32 much more clearly than in FIG. 31.
FIG. 33 shows the configuration of an optical transmission system utilized in the related art to eliminate effects from the non-linear optical effect. This system contains one standard clock supply device 51. The clock signals 52 are output from the standard clock supply device 51 on an identical frequency and identical phase and are supplied to a plurality of transmit side optical transmission devices 531, 532, . . . 53N. These transmit side optical transmission devices 531, 532, . . . 53N each contain a clock interface (clock I/F) 55, a frame processor 56 input with frame pulses 56 from these transmit side optical transmission devices 531, 532, . . . 53N, and an electrical-optical converter (E/O) 58.
The clock interfaces 55 are circuits comprised of frequency dividers to convert the clock signal 52 supplied from the standard clock supply device 51 to a clock frequency signal appropriate for its own transmit side optical transmission devices 53. The respective transmit side optical transmission devices 531, 532, . . . 53N require their own unique clock frequency for occasions when discrepancies exist between the signal processing contents themselves or the manufacturer. The frame processor 57 processes the externally input data signals 59 by utilizing the frame pulses 56. The electrical signals 61 of the frame format that was generated are input to the electrical-optical converter (E/O) 58 and converted to optical signal 621. Signal processing results from respective transmit side optical transmission devices 531, 532, . . . 53N are converted to optical signals 621, 622 . . . 62N in the same way and output.
These optical signals 621, 622 . . . 62N are input to the transmit side WDM device 64. The transmit side WDM device 64 contains optical interfaces 641, 642, . . . 64N to input the respective optical signals 621, 622, . . . 62N a wavelength multiplier 65 located at the output of these interfaces 64, and an optical amplifier 671, to amplify the optical signals 66 multiplexed in the wavelength multiplier 65. The optical interfaces 641, 642, . . . 64N are set beforehand with mutually different delay times for the received optical signals 621, 622, . . . 62n so that the phase of the bit patterns of the respective overhead portions are mutually offset from each other.
An optical signal of wavelengths 21 . output from the optical amplifier 67 is input to the optical amplifier 67(k+1) of the receive side WDM device 73 by way of optical transmission path fibers 711, . . . 71K, 71(K+1) and the optical amplifiers 672, . . . 67K, and then isolated into the optical signals 741, 742 . . . 74n, of respective wavelengths xcex1 . . . xcexN in a wavelength isolator 74. These optical signals 741, 742 . . . 74n are converted into data signals optical signals 761, 762 . . . 76n, and supplied by way of receive side optical transmission device 751, 752 . . . 75n, to a portion of a circuit in a latter stage not shown in the drawing.
A precondition for such kind of optical transmission systems of the related art was that the respective transmit side optical transmission devices 531, 532, . . . 53N, shown in FIG. 33 have exactly the same electrical characteristics. Under this condition, the bit pattern of the overhead portion of the respective optical signals were made to have different phases by a method for instance, for increasing the amount of delay by several sequential bits. The composition of the data of the optical signals sent by the respective transmit side optical transmission devices 531, 532, . . . 53N were of course then different, but the overhead portion placed prior to these data portions in many cases had patterns identical or only slightly different from the bit patterns. So in order to suppress the non-linear optical effect and reproduce a satisfactory overhead portion of these bit patterns, the positions of the respective overhead portions were offset or displaced over time.
In actual fact however, as previously explained, the individual transmit side optical transmission devices 531, 532, . . . 53N do not necessarily always have identical electrical characteristics. To the contrary, the electrical characteristics are in many cases different due to variations between the manufacturers. So even therefore, assuming no measures are taken to delay the phase of the transmit side optical transmission devices 531, 532, . . . 53N and that all process the overhead and the following data at absolutely the same timing, the mutual time position of the respective optical signals, or in other words, the phase will usually very to a small degree, after passing through these transmit side optical transmission devices 531, 532, . . . 53N.
Consequently, even if measures is taken on purpose, to provide a phase delay means to delay the phase for each of the transmit side optical transmission devices 531, 532, . . . 53N, if the amount of phase delay occurring due to this delay means and the amount of delay due to individual characteristics of each of the transmit side optical transmission devices 531, 532, . . . 53N are identical amounts with different plus and minus signs, then bit pattern phase differentials may cancel each other out causing a phase of zero or near zero to occur. In such a case, that pair of overheads will be susceptible to the non-linear optical effect and reproducing that data is difficult.
Further, even assuming that, an effect is obtained so that the overhead bit pattern portions have mutually different phases at that time, replacement of some of those transmit side optical transmission devices 531, 532, . . . 53N will prove necessary at some point. Replacement brings the possibility that optical signals will be reproduced in which the bit pattern phases essentially match each other, so signals with poor transmission characteristics will likely be transmitted due to the non-linear optical effect.
Japanese Patent Laid-open 7-66779 (published patent) also discloses a measure to suppress the non-linear optical effect so that method is briefly explained and the objective of this invention clarified.
FIG. 34 is a drawing showing the overall structure of that disclosed optical transmission system. This system is comprised of a transmit station 81, a receive station 82, optical fibers 831, 832, . . . arrayed in series to connect between the stations 81 and 82, optical amplifiers 841, 842, . . . , and optical group dispersion compensating fibers 851, 852, . . . 85N. The transmit station 81 is comprised a plurality of optical transmitters 861, 862, . . . , and optical coupler 87 to couple the optical signals of different wavelengths sent from the optical transmitters 861, 862, and an optical amplifier 88 to amplify the coupled optical signals. The receive station 82 is comprised of an optical divider 91 to divide the optical signals received from the receive station 82, and receivers 921, 922, . . . to receive the respective optical signals of different wavelengths after division.
In this disclosed optical transmission system, each bit of a bit pattern is generated at a time difference one-half or more that of the other bit so that the mutual phase modulation effect between bits is suppressed.
FIGS. 35(a) to 35(d) show the signal processing in a system preceding the previous disclosed optical transmission system. In this preceding disclosed system, the optical group dispersion compensating fibers 851, 852, . . . 85N of the previous proposed optical transmission system of FIG. 34 are omitted and a direct connection made. FIG. 35(a) shows the transmit signal waveform for one of the optical transmitters 861, shown in FIG. 34. A precondition is that modulation of the transmit signal be NRZ (Non Return To Zero) modulation. The xe2x80x9c0xe2x80x9d or the xe2x80x9c1xe2x80x9d shown on the horizontal axis indicate the status of the respective bit.
FIG. 35(b) shows XPM (Cross Phase Modulation) frequency chirp brought about by an optical signal such as shown in FIG. 35(a). This waveform is the optical intensity waveform differential of the optical signal shown in FIG. 35(a). Therefore, in this signal waveform, the rising edges on this waveform of FIG. 35(a) are negative and falling edges are positive.
FIG. 35(c) shows the waveform of the optical signal of another one optical transmitters 862 shown in FIG. 34. FIG. 34(d) shows the waveform received from the receive station 82 output from these two optical transmitters 861, 862. As shown in this FIG. 34(d), the high value of this received waveform has large fluctuations and the waveform changes greatly so that accurately isolating these signals is impossible.
FIGS. 36(a) to 36(d) show the signal processing in the disclosed optical transmission system. In this method, the optical signals are delayed by respective one bit periods by means of the optical group dispersion compensating fibers 851, 852, . . . 85N shown in FIG. 34. Therefore, the XPM frequency chirp (B of same figure) generated inside the optical fiber 831, and the XPM frequency chirp (C of same figure) generated inside the optical fiber 832 in the waveform of the transmit signal of another one of the optical transmitters 862 such as shown in FIG. 36(a), are slightly offset on the time axis to have a interval of xc2xd bit or more. The waveform of the receive station 82 from the transmit signal of another one of the optical transmitters 862 becomes a waveform such as shown in FIG. 36(d) the distortion of the waveform as seen in bit units becomes small, and these signals can be satisfactorily received and reproduced.
In the disclosed optical transmission systems as described in FIG. 34 through FIGS. 36(a) to 36(d), alleviation of the non-linear optical effect was attempted by changes in each one bit of data and is therefore basically different from technology to resolve the non-linear optical effect in entire bit patterns comprised of a plurality of bits.
In view of the above problems with the related art, this invention has the object of providing an optical transmission system to reduce the non-linear optical effect in overhead portions of bit patterns comprised of a plurality of bits.
According to first aspect of the invention, an optical transmission system has; (A) a plurality of transmit side optical transmission devices to make respectively unique frames having the same frame period based on an identical standard clock, add an overhead of specified length to the beginning of these frames and send as respectively different optical signals and, (B) a wavelength-division multiplexing signal transmission means to perform wavelength-division multiplexing of optical signals of different wavelengths sent from a plurality of transmit side optical transmission devices, and send to a device on the receive side by way of the transmission path and, (C) an overhead phase alignment means to store beforehand the time offsets of the frames generated using said standard clock at said plurality of transmit side optical transmission devices, and set the amount of time delay of optical signals output from said plurality of transmit side optical transmission devices so that the overheads at each of these transmit side optical transmission devices are provided with a mutual time offset and wavelength multiplexed.
In other words, first aspect of the invention aligns the phase of overheads of optical signals output from a wavelength-division multiplexing signal transmission means from signals whose frames start at essentially a completely identical timing based on the same clock frequency. The frames that are generated have time differentials due to differences among manufacturers of the optical transmission devices and the circuits being used. These frame time differentials are compensated for and the phase of this signals aligned so that overheads will be mutually provided with a time offset and wavelength multiplexed. The overhead phase alignment means therefore stores beforehand the time offset of that frames that are generated when a standard clock is used at each of the plurality of transmit side optical transmission devices, and sets a delay in the optical signal output from these transmit side optical transmission devices so that the overheads will have a mutual time offset and are wavelength multiplexed.
Therefore, according to first aspect of the invention, there is no need to determine the respective overhead positions of the optical signals actually sent from the plurality of transmit side optical transmission devices, and the overhead positions for the respective optical signal wavelengths can be easily offset, so that the non-linear optical effect can be easily reduced.
According to second aspect of the invention, an optical transmission system has; (A) a plurality of transmit side optical transmission devices to make respective unique frames based on identical standard clocks, add an overhead of specified length to the beginning of these frames and send them as respectively different optical signals and, (B) a phase detection means to detect the time offsets of overheads in optical signals sent from the plurality of transmit side optical transmission devices and, (C) an overhead phase alignment to set the amount of time delay for optical signals output from the plurality of transmit side optical transmission devices based on results from said discrimination means, so that overheads at each or the transmit side optical transmission devices have a mutual time offset and are wavelength multiplexed and, (D) and a wavelength division multiplexing means to receive the optical signals with delays adjusted by the overhead phase alignment means from the plurality of transmit side optical transmission devices, and perform wavelength division multiplexing of the signals of different wavelengths, and send along a transmission path to a receive side device.
In other words, second aspect of the invention differs from the first aspect of the invention in that a phase detection means detects the time offsets of overheads in optical signals sent from the plurality of transmit side optical transmission devices, and sets the delay amount of the optical signal sent from the transmit side optical transmission devices so that the chase of the overhead will be offset according to the detected results. Feedback control is thus performed in this way so that even if a completely new transmit side transmission device is installed, or the electrical characteristics of an already used transmit side transmission device change, the offset in the overhead of these optical signals that were output will still be correctly set and the non-linear optical effect can be alleviated.
According to third aspect of the invention, the optical transmission system has; (A) a plurality of transmit side optical transmission devices to make respective unique frames based on identical standard clocks, add an overhead of specified length to the beginning of these frames and send them as respectively different optical signals and, (B) a phase detection means to detect the time offsets of overheads in optical signals sent from the plurality of transmit side optical transmission devices and, (C) an overhead phase alignment to set the amount of time delay for optical signals based on results it from the detection means, so that overheads at each of the transmit side optical transmission devices have a mutual time offset and are wavelength multiplexed and, (D) a wavelength division multiplexing means to perform wavelength division multiplexing of the optical signals with delays adjusted by the overhead phase alignment means and send along a transmission path to a receive side device.
In other words, third aspect of the invention differs from the first aspect of the invention in that a detection means detects the time offsets of overheads in optical signals sent from the plurality of transmit side optical transmission devices, and sets the delay amount of the optical signal sent from the transmit side optical transmission devices so that the phase of the overhead will be offset according to the phase detection results. Third aspect of the invention differs from the third aspect of the invention in that the delay amount is not set at the transmit side optical transmission device end, and the amount of delay in the optical signal sent from the transmit side optical transmission devices is adjusted at each wavelength before performing wavelength-division multiplexing. Feedback control is thus performed in this way so that even if a completely new transmit side transmission device is installed, or the electrical characteristics of an already used transmit side transmission device change, the offset in the overhead of these optical signals that were output will still be correctly set and the non-linear optical effect can be alleviated. There is also no need to set the amount of delay at the transmit side optical transmission device end so the circuit has a simpler structure.
According to fourth aspect of the invention, the optical transmission system has; (A) a plurality of transmit side optical transmission devices to make respective unique frames based on identical standard clocks, add an overhead of specified length to the beginning of these frames and send them as respectively different optical signals and, (B) a phase detection means to determine the time offsets of overheads in optical signals sent from the plurality of transmit side optical transmission devices and, (C) a wavelength-division multiplexing means to perform wavelength division multiplexing of the optical signals from said phase detection mean, and (D) a selectable phase dispersion means to select a desired dispersion fiber from a plurality of dispersion fibers of different dispersion values for differing wavelengths and, (E) a selection means to select a dispersion fiber having the most effect in reducing the non-linear optical effect in optical signals which is launched into transmission fiber as wavelength multiplexed signals.
In other words, fourth aspect of the invention, determines the time offset of the overhead in the optical signal sent from the plurality of transmit side optical transmission devices, and controls a switching means so that dispersion fibers can be selected for optimum placement of wavelength offsets. The overheads can be placed as needed without having to use a special delay means, and the non-linear optical effect can be reduced.